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make falls back to the GNU toolchain when clang is absent, and that works for the kernel. It does not work for the UEFI loader: BOOTX64.EFI is PE/COFF and wants clang plus lld-link, so a machine with only gcc gets a kernel that builds and a loader that silently stays whatever it already was. That is not hypothetical -- it is what left a stale boot splash on this tree for half a day. The mask in include/logo.h was the new mark, the kernel drew it, and the loader kept drawing one compiled months earlier because nothing could relink it and nothing said so. shell.nix declares everything docs/guides/building.md lists, and building.md gains a NixOS section saying why using it matters rather than just how. Verified: with clang 21.1.8 and LLD 21.1.8 the loader relinks and the splash draws Shift in metal, screendumped from the framebuffer rather than inferred. Pinned to the host's nixpkgs rather than a flake input, deliberately. This is a convenience for a development machine, not a reproducibility claim; if it ever needs to be the latter it wants a flake and should say so. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2
144 lines
5.9 KiB
Markdown
144 lines
5.9 KiB
Markdown
<!-- SPDX-License-Identifier: Apache-2.0 -->
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# Building SchOS
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Supported hosts: **Linux, macOS, FreeBSD/OpenBSD/NetBSD, and Windows** (under
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Git Bash or MSYS2). The build is developed from both Windows and Linux, so a
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change that only works on one of them is a bug.
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```sh
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make # kernel + UEFI loader + root filesystem
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make qemu # build and boot it
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```
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## Prerequisites
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| Need | Why | Windows | Linux | macOS | BSD |
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|---|---|---|---|---|---|
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| `clang`, `ld.lld`, `llvm-objcopy`, `llvm-objdump` | cross-compiles the kernel, the 16/32-bit boot code, and musl userland | LLVM installer | `clang lld llvm` | `brew install llvm` | `pkg install llvm` |
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| `make` | — | Git Bash | `make` | Xcode CLT | `gmake` |
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| `perl` | generates the interrupt vector table | Git Bash | usually present | present | `pkg install perl5` |
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| `python3` | boot logo, crash-screen QR | python.org (`python`) | `python3` | `brew install python` | `pkg install python3` |
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| `qemu-system-x86_64` | running and testing | `scoop install qemu` | `qemu-system-x86` | `brew install qemu` | `pkg install qemu` |
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| `curl`, `tar` | fetching ports | Git Bash | present | present | present |
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| `xorriso` **or** `mkisofs` | `make iso` only | `scoop install cdrtools` | `xorriso` | `brew install xorriso` | `pkg install xorriso` |
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On NixOS, `nix-shell` supplies every row above — see below.
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## NixOS
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```sh
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nix-shell # then: make, make qemu, make iso
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```
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`shell.nix` in the repository root declares the lot. Worth using rather than
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assembling it by hand, for one specific reason: **`make` falls back to the GNU
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toolchain for the kernel but the UEFI loader genuinely cannot be built without
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clang and `lld-link`.** A machine with only gcc therefore produces a kernel that
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builds and a loader that silently stays whatever it already was — which left a
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stale boot splash on this tree for half a day before anyone looked at the
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screen rather than the build output.
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PowerShell is the one thing the shell does not supply, and `tools/windows/`
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needs it. See the note at the end of this file.
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A host C compiler (`cc`) is also needed, for the tools that build the disk
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images. On Windows that must be **mingw-w64's gcc**, not clang: clang defaults
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to an MSVC target there, and `tools/mkfs.c` needs `<unistd.h>` and `<dirent.h>`,
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which the Windows SDK does not have.
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## Why clang rather than a cross-gcc
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clang is a cross-compiler by construction, so one ordinary LLVM install targets
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everything this tree needs — bare-metal ELF, code linked below one megabyte, and
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Linux/musl userland — from any host. A cross binutils+gcc would be a day of
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setup per platform that then rots, and on macOS it means brewing
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`x86_64-elf-gcc` before anything builds at all.
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The Makefile selects clang+lld whenever both are on `PATH`. To force the GNU
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toolchain instead:
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```sh
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make USE_GNU=1 # needs a cross toolchain that emits x86_64 ELF
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make USE_GNU=1 TOOLPREFIX=x86_64-elf-
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```
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## Testing
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```sh
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tools/windows/bootcheck.ps1 # boots, drives the shell, asserts on output
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tools/windows/bootcheck.ps1 -Tty -Ps # plus terminal behaviour and process listing
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make schfs-check # SchFS unit, object and image suites
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```
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`bootcheck` is PowerShell, and PowerShell 7 (`pwsh`) runs on Linux, macOS and
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the BSDs as well as Windows — `brew install powershell`, or your distribution's
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`powershell` package. It finds QEMU on `PATH`.
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Booting to a prompt proves nothing on its own: a filesystem can mount and still
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fail the first real command, which has happened here. `bootcheck` types commands
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at the serial console and greps the transcript for the answers they should
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produce.
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## Ports
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`musl`, `toybox` and `mksh` are built from source into `sysroot/`. **This does
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not work on a Windows host** — `make` prints `ports: skipped` and uses whatever
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is already staged in `sysroot/`. Building them needs a Linux host (or any other
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Unix); do it once and the staged result serves the Windows builds.
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## Where the tools live
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`tools/` is grouped by what runs the file, so it is obvious at a glance which
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tools a given host can use:
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```
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tools/
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windows/ .ps1 PowerShell (pwsh runs these on Linux and macOS too)
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posix/ .sh shell
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python/ .py
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*.c host programs -- compiled by the build, run everywhere
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*.pl perl, likewise
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```
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The `.c` and `.pl` files stay at the top level because they are not
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host-specific: the build compiles or invokes them on every platform.
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A script that needs the repo root derives it from its own location, so anything
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moved between these directories has to have that adjusted — the PowerShell tools
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use `Split-Path (Split-Path $PSScriptRoot -Parent) -Parent`, two levels up.
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## Known host differences, already handled
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Listed because they are the shape of the next such bug, not because anything is
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outstanding:
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- `<sys/sysmacros.h>` is glibc/musl only. macOS and the BSDs take `major()` and
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`minor()` from `<sys/types.h>`.
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- `sha256sum` exists on Linux and Git Bash; macOS has `shasum -a 256`,
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FreeBSD/NetBSD `sha256`, OpenBSD `cksum -a sha256`. `tools/posix/sha256check.sh`
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finds whichever is present.
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- `nproc` is coreutils, so absent on macOS and BSD, where it is
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`sysctl -n hw.ncpu`.
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- In the PowerShell tools, path literals must use forward slashes:
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`Join-Path $root 'build\x'` produces a *filename containing a backslash* on a
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Unix host.
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## Fedora, in one command
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```sh
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tools/posix/setup-fedora.sh --check # what is missing
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tools/posix/setup-fedora.sh # install it
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```
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Moving an existing checkout to a Linux machine: see
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[../history/moving-to-linux.md](../history/moving-to-linux.md).
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## Working on two machines
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Claude Code's project memory is kept in the repo and moved with:
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```sh
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tools/posix/memory-sync.sh status
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tools/posix/memory-sync.sh export # after a session that learned something; then commit
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tools/posix/memory-sync.sh import # after cloning or pulling elsewhere
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```
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